Some of the roadblocks that have limited successful commercialization of deep UV devices are high dislocation densities, nonradiative defects and, particularly, low hole concentrations and poor hole injection. Large activation energies and, therefore, poor efficiency of the Mg acceptor in GaN and AlGaN has delayed maturity of these devices. The resistivities of bulk p-GaN and p-AlxGa1-xN are high. Activation energy of Mg dopants and concentration of compensating defects also increase with x in AlxGa1-xN, presenting a challenge for the growth of low resistance material for shorter wavelength applications. LEDs often employ a p-GaN capping layer as a less resistive contact layer, but at the expense of severe optical absorption losses at UV wavelengths. The removal of the p-GaN capping layer and significant improvement in p-type doping and hole injection will need to be achieved in order to realize high performance deep UV devices.
Superlattices (SLs) comprise two semiconductor materials with different band gaps which are deposited alternately on each other to form a periodic structure in the growth direction. Recently, SLs have been investigated as replacements for bulk AlxGa1-xN to increase the hole concentration within the p-type layer. Spontaneous polarization is inherent in the nitride materials grown in the (0001) direction due to their wurtzite crystal structure. This allows unique device structures to be developed. Power devices such as high electron mobility transistors (HEMTs) have relied upon the AlGaN/GaN heterostructure which forms a two-dimensional electron gas (2DEG) at the interface. The 2DEG is a thin, highly conductive, lateral channel from which HEMTs can greatly benefit. Superlattices of alternating p-type nitride layers enjoy a similar carrier accumulation at the interfaces, producing parallel sheets where band bending increases the ionization of acceptors and therefore increases the concentration of holes, forming a two-dimensional hole gas (2DHG). See P. Kozodoy et al., Appl. Phys. Lett. 74, 3681 (1999); E. F. Schubert et al., Appl. Phys. Lett. 69, 3737 (1996); and K. Kumakura et al., Jpn. J. Appl. Phys. 39, 2428 (2000).
Superlattice structures have been integrated into GaN-based LEDs and LDs to replace the bulk p-type AlGaN with some success. See S.-N. Lee et al., J. Cryst. Growth 287, 554 (2006); S. A. Nikishin et al., Jpn. J. Appl. Phys. 42, L1362 (2003); T. Nishida et al., Jpn. J. Appl. Phys. 42, 2273 (2003); and S. A. Nikishin et al., Jpn. J. Appl. Phys. 44, 7221 (2005). Measured resistivities of these p-type SLs are similar to that of p-GaN layers, but the Hall measurement only determines lateral resistivity. See P. Kozodoy et al., Appl. Phys. Lett. 74, 3681 (1999); and S. A. Nikishin et al., Jpn. J. Appl. Phys. 42, L1362 (2003). The measured or estimated vertical resistivity of various p-SL stacks is typically greater than the horizontal resistivity. See S. A. Nikishin et al., Jpn. J. Appl. Phys. 42, L1362 (2003); C. Y. Hu et al., J. Cryst. Growth 298, 815 (2007); and M. Z. Kauser et al., Mater. Res. Soc. Symp. Proc. 831, E3.39.1 (2005). Although development of p-type SLs has increased the spatial carrier concentration compared to bulk p-type material, there is still a high vertical resistivity to overcome suggesting that the conventional vertical LED or LD design may be limiting forward progress in this area.
A number of groups have reported lateral LEDs and LDs in III-V systems other than nitrides. See V. Ryzhii et al., J. Appl. Phys. 90, 2654 (2001); V. Ryzhii et al., J. Appl. Phys. 92, 4459 (2002); S. M. Komirenko et al., Solid-State Electron. 47, 169 (2003); M. Cecchini et al., Appl. Phys. Lett. 82, 636 (2003); A. North et al., IEEE J. Quantum Electron 35, 352 (1999); E. H. Sargent, Solid-State Electron. 44, 147 (2000); and U.S. Pat. No. 5,563,902. Other groups have reported the use of a p-SL structure in a nitride LED or LD, but their designs still employ a vertically injected p-i-n structure where current flow is perpendicular to the layers in the superlattice. See M. Shatalov et al., Applied Physics Express 5 082101 (2012), S.-N. Lee et al., J. Cryst. Growth 287, 554 (2006); S. A. Nikishin et al., Jpn. J. Appl. Phys. 42, L1362 (2003); T. Nishida et al., Jpn. J. Appl. Phys. 42, 2273 (2003); G. Kipshidze et al., J. Appl. Phys. 93, 1363 (2003); T. Nishida et al., Appl. Phys. Lett. 78, 399 (2001); S. Nakamura et al., Appl. Phys. Lett. 76, 22 (2000); S. Nakamura et al., Appl. Phys. Lett. 72, 211 (1998); and S-N. Lee et al., Appl. Phys. Lett. 88, 111101 (2006).
Therefore, a need remains for a laterally-injected III-nitride LED or LD having a p-SL hole injection region.